Several configurations for connecting floating structures with a seabed pipeline or wellhead system for the purposes of fluid transfer therebetween are known in the art, and are commonly used in the offshore oil and gas industry.
One frequently used configuration is known as the free-hanging catenary configuration, often called a Simple Catenary Riser or SCR. Such a riser configuration is illustrated in FIG. 1 and includes a riser pipe 10 freely hung from a host vessel 12, such as a FPSO vessel, and forming a curved shape downwards until it lands on the seabed 14 at a touchdown point 16. After the touchdown point 16, the pipe horizontally lies on the seabed 14 and connects to subsea facilities, such as subsea hydrocarbon production facilities and infrastructure (not illustrated in FIG. 1). In this configuration, and regardless of the type of riser pipe 10 used, the oscillations of the vessel 12 may induce high curvature fluctuations of the pipe in the lower part of the riser, especially in the region of the touchdown point 16. This curvature can overstress the pipe and additionally may lead to significant fatigue-damage in the vicinity of the touchdown point of the riser.
When a riser, in this free-hanging configuration, consists of a rigid tube formed of metal such as steel, the radius of curvature at the touchdown point is made relatively large to minimise the possibility of stress exceeding the yield strength of the metallic pipe material. However, this may result in the requirement to use longer lengths of riser pipe, which can significantly increase the weight of the riser, giving rise to additional problems, such as exceeding vessel deck load limits and the like. Furthermore, this free hanging catenary configuration is highly sensitive to fatigue damage accumulation particularly at the welds used to connect the individual metallic pipe sections together.
A method for optimising the response in metal risers of this known catenary form is to apply buoyancy modules along the near horizontal section of the riser to favourably modify its response in the vicinity of the touchdown point. Varying quantities and distributions of buoyancy may be considered from small amounts that only provide a small upthrust and almost imperceptible change in curvature, to larger quantities that can result in large sections of pipe being vertically lifted off the seabed to form a riser shape that is often referred to as a wave catenary. Such a wave catenary form is illustrated in FIG. 2, wherein a riser pipe 20 is again hung from a vessel 22 and extends to the seabed 24. A section of the riser pipe 20 includes buoyancy modules 26, such as syntactic foam and/or aircans, which establish a wave configuration 28 along the length of the riser pipe 20. This wave configuration 28 assists to largely decouple the effect of motion of the vessel 22 from the riser pipe 20 at the region of a touchdown point 30 with the seabed 24, thus assisting to minimise stress and fatigue in this region.
A flexible pipe made from alternating layers of helically wound steel and thermoplastic materials may be used in deep seas in the free-hanging configuration. Such layered flexible pipe is typically known as non-bonded pipe in the art. Such flexible non-bonded pipe, when used in the SCR form, may have advantages over metallic equivalents, for example in that a smaller radius of curvature at the touchdown point may be permissible. Furthermore, flexible pipe may allow greater vertical and horizontal movements of the host vessel at the water surface due to smaller allowable bend radii and improved fatigue behaviour. However, known flexible pipe may have the drawbacks of being very heavy, exhibiting inferior thermal insulation, and having a higher cost per unit length than steel equivalents.
A further riser configuration uses the combination of buoyancy modules attached to the riser pipe to form an arch or wave in combination with a tensioned seabed tether which anchors a point on the riser below the wave to a fixed point on the seabed. This tether is assembled from steel wires or chain and is used to control riser shape and deflections. This configuration is commonly referred to as a Pliant wave. A development of the Pliant wave arrangement is proposed in WO 2009/139636.
A further wave catenary configuration is disclosed in US 2009/0269141 which proposes a combination of tethers and buoyancy modules wherein the tether is connected under tension between a point of fixity on the seabed and a point on the riser that is coincident with the point of application of the buoyancy modules.
The foregoing SCR and Wave catenary risers are primarily applicable to metallic steel pipe risers and non-bonded flexible pipe risers. These pipe constructions are often heavy and result in high tensions and payloads on the host vessel. However, the benefit of such high tensions is that they assist the stability of the riser structure to resist the application of hydrodynamic current forces.
Free standing or hybrid risers are also often used in the oil and gas industry to transfer fluids from surface vessels to and from subsea wellheads and pipelines. Free standing risers are typically used in deep water and comprise a long, stiff and largely vertical lower section which is quasi static, and a shorter flexible near surface upper section configured in a free hanging catenary configuration. The catenary upper section, typically called a flexible jumper, is designed to accommodate vessel motions and is typically constructed from non-bonded flexible pipe. As noted above, non-bonded flexible pipe has the benefit that it can accommodate small bend radii and this along with its relatively heavy in-water weight allows acceptable configurations to be achieved even when vessel motions and mooring excursions are large. However the disadvantage is that its weight can be excessive and this can detrimentally add to vessel and riser payload. Additionally, non-bonded flexibles have further limitations on their maximum diameter, maximum service temperature, sour service acceptability and long term robustness.
Generally, subsea pipelines, whether extending from surface to seabed, or simply extending entirely subsea, may suffer from similar issues to those identified above, such as requirement to minimise regions of high stress and fatigue, control of dynamic response to environmental conditions, excessive weight and the like.